1. Field of the Invention
The present invention generally relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices are widely used in xe2x80x98image forming apparatusxe2x80x99 such as a digital copier, an optical printer, an optical plate-making machine, a facsimile machine and so forth. A writing density of optical scanning devices has been increased to 1200 dpi, 1600 dpi, and is intended to be increased, further higher.
In order to achieve such high-density writing, it is necessary to form a beam spot having a small diameter, and, also, quality and stability of a beam spot is needed to be improved. Stability of a beam spot is determined from determining whether or not xe2x80x98a variation in a beam-spot diameter on a surface to be scanned due to a variation in an image heightxe2x80x99 is very small and stable. Quality of a beam spot is determined from determining whether or not xe2x80x98the light-intensity distribution of a beam spot has a simple mountain shape and does not have a complicated lower slope shapexe2x80x99.
In order to achieve a beam spot having high-quality and stability, it is necessary for a scanning and image-forming optical system of an optical scanning device to have a high performance for forming a beam spot on a surface to be scanned using a deflected light flux. A factor causing a beam-spot diameter to fluctuate is, as is well known, xe2x80x98curvature of field in a scanning and image-forming optical systemxe2x80x99, and many scanning and image-forming optical systems in which curvature of field is well corrected have been proposed. Further, it is important for an optical magnification in a scanning and image-forming optical system to be fixed when an image height of a beam spot changes.
However, in order to form a beam spot having stability and high quality, not only it is necessary to correct an optical performance such as curvature of field and an optical magnification but also it is important to xe2x80x98set a wave-optical wavefront aberration to be fixed between respective image heightsxe2x80x99.
An object of the present invention is to achieve high-density, satisfactory optical scanning with a stable and high-quality beam spot, by well correcting not only curvature of field and optical magnification but also xe2x80x98wavefront aberration on pupilxe2x80x99 in a scanning and image-forming optical system.
An optical scanning device according to the present invention is xe2x80x98a device which deflects one or a plurality of light flux(es) originating from a light source by an optical deflecting unit, gathers the deflected light flux(es) to cause it (them) to form a beam spot(s) on a surface to be scanned by a scanning and image-forming optical system, and, thus, performs optical scanning of the surface to be scannedxe2x80x99.
As one or a plurality of light flux(es) is (are) emitted from the light source, the optical scanning device according to the present invention can be put in to practice either as an optical scanning device in a single-beam-scanning system in which optical scanning is performed using a single beam spot or an optical scanning device in a multi-beam-scanning system in which a plurality of scan lines are scanned simultaneously by a plurality of beam spots.
The scanning and image-forming optical system includes one or a plurality of optical component(s) including a lens. Accordingly, the scanning and image-forming optical system may include, other than the lens, xe2x80x98a reflecting-surface component having a mirror surface having an image-forming functionxe2x80x99.
Further, at least one surface of the lens included in the scanning and image-forming optical system is a sub-non-arc surface.
The xe2x80x98sub-non-arc surfacexe2x80x99 is a surface having an arc or non-arc shape in a main scanning plane, and having a non-arc shape in a sub-scanning plane.
The xe2x80x98main scanning planexe2x80x99 is a plane including the optical axis of the lens and parallel to main scanning directions in the lens or in the vicinity thereof.
The xe2x80x98sub-scanning planexe2x80x99 is a plane perpendicular to the main scanning directions in the lens or in the vicinity thereof.
The optical scanning device according to the present invention is characterized in that the sub-non-arc surface is formed in xe2x80x98a lens in which a diameter of a light flux passing through the scanning and image-forming optical system is largest in the sub-scanning planexe2x80x99.
That is, when the scanning and image-forming optical system has xe2x80x98one sub-non-arc surfacexe2x80x99, this sub-non-arc surface is formed in the above-mentioned xe2x80x98lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning planexe2x80x99, and, when the scanning and image-forming optical system has two or more sub-non-arc surfaces, at least one thereof is provided in the above-mentioned xe2x80x98lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning planexe2x80x99. In this case, both the surfaces of this lens may be sub-non-arc surfaces.
In this optical scanning device, the sub-non-arc surface may be a surface of the xe2x80x98lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning planexe2x80x99, xe2x80x98in which surface a diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning planexe2x80x99.
The optical scanning device according to another aspect of the present invention is characterized in that a sub-non-arc surface is formed in xe2x80x98a lens having the largest effective diameter in the main scanning planexe2x80x99.
In this case, the sub-non-arc surface may be a surface of the lens, which surface has the largest effective diameter in the main scanning planexe2x80x99.
Degradation in wavefront aberration is likely to occur as a wave surface is large. Accordingly, a surface of a lens through which correction of wavefront aberration can be effectively made is a portion at which a wave surface is large, and, therefore, a surface of each of the above-mentioned lenses or each of the above-mentioned surfaces of the lenses is suitable for having a sub-non-arc surface through which wavefront aberration is corrected. Further, in such a portion as that in which a wave surface is large, a sub-non-arc surface itself is large, and, thereby, it is easy to form the sub-non-arc surface.
The optical scanning device according to another aspect of the present invention is characterized in that the sub-non-arc is formed in xe2x80x98a lens of a scanning and image-forming optical system having a surface in which, throughout an effective range of the lens, the incidence angle of the chief ray of a deflected light flux incident on the respective surfaces of the lens is equal to or less than 25 degreesxe2x80x99.
In this case, a xe2x80x98surface of the lens, in which surface, in which, throughout the effective range of the lens, the incidence angle of the chief ray of a deflected light flux incident on the respective surfaces of the lens is equal to or less than 25 degreesxe2x80x99 may be formed to be the sub-non-arc surface.
In a surface in which the incidence angle is larger than 25 degrees, a refractive index of this surface in a main scanning plane is large. When such a surface is formed to be the sub-non-arc surface, it is not necessarily easy to achieve both an effect of correction of wavefront aberration in the sub-scanning directions and an effect of correction of characteristics in the main scanning directions. Accordingly, it is preferable for a xe2x80x98surface, in which the incidence angle of the chief ray of a deflected light flux is equal to or less than 25 degrees throughout an effective range of the lensxe2x80x99, to be the sub-non-arc surface.
The optical scanning device according to another aspect of the present invention is characterized in that the sub-non-arc surface expressed by a coordinate X(Y, Z) in the optical-axis direction is expressed by the following equation:
X(Y, Z)=CmY2/[1+{square root over ({1+L xe2x88x92(1+K+L )Cm2Y2+L })}]+xcexa3AnYn+Cs(Y)Z2/[1+{square root over ({1xe2x88x92+L (1+Kz+L (Y+L ))Cs2+L (Y+L )Z2+L })}]+fSAG(Y, Z)xe2x80x83xe2x80x83(1)
where xe2x80x98Yxe2x80x99 denotes a coordinate in the main scanning direction, xe2x80x98Zxe2x80x99 denotes a coordinate in the sub-scanning direction, xe2x80x98Cmxe2x80x99 denotes a paraxial curvature in the main scanning directions on the optical axis or in the vicinity thereof, xe2x80x98Cs(0)xe2x80x99 denotes a paraxial curvature in the sub-scanning directions on the optical axis or in the vicinity thereof, xe2x80x98Cs(Y)xe2x80x99 denotes a curvature in the sub-scanning plane at a coordinate Y in the main scanning direction, xe2x80x98Kxe2x80x99 denotes a conical constant of a quadric curve in the main scanning plane on the optical axis, xe2x80x98Kz(Y)xe2x80x99 denotes a conical constant of a quadric curve in the sub-scanning plane at a coordinate Y in the main scanning direction, and xe2x80x98fSAG(Y, Z)xe2x80x99 denotes a non-spherical-surface high-order correction amount.
The sum of the second term is taken for n from n=1 to n=p (desired order number).
In this optical scanning device, the above-mentioned curvature Cs(Y) can be expressed by the following equation:
Cs(Y)={1/Rs(0)}+B1Y+B2Y2+B3Y3+B4Y4+ . . . xe2x80x83xe2x80x83(2)
using a radius Rs(0) of paraxial curvature in the sub-scanning plane on the optical axis or in the vicinity thereof, and constant coefficients B1, B2, B3, . . . .
Further, in the above-mentioned optical scanning device, the above-mentioned conical constant Kz(Y) can be expressed by the following equation:
xe2x80x83Kz(Y)=C0+C1Y+C2Y2+C3Y3+C4Y4+ . . . xe2x80x83xe2x80x83(3)
using constant coefficients C0, C1, C2, C3, . . . .
Furthermore, in the above-mentioned optical scanning device, the above-mentioned high-order correction amount fSAG(Y, Z) can be expressed by the following equation
fSAG(Y, Z)=xcexa3(xcexa3dj, hYh)Zjxe2x80x83xe2x80x83(4)
using constant coefficients dj, h.
The sum of the right side is taken for h from h=0 to h=q (desired order), and for j, j=1 to j=r (desired order).
In each of the above-mentioned optical scanning devices, it is possible for the optical deflecting unit to be xe2x80x98a unit (polygon mirror, rotational single-surface mirror, rotational two-surface mirror or the like) which deflects a light flux(es) originating from the light source at a uniform angular velocityxe2x80x99, and for the scanning and image-forming optical system to be xe2x80x98a system having a function of making a velocity of optical scanning by the light flux(es) deflected at the uniform angular velocity be uniformxe2x80x99.
Further, in each of the above-mentioned optical scanning devices, the lens of the scanning and image-forming optical system having the sub-non-arc surface may be made of xe2x80x98a plastic materialxe2x80x99. For manufacturing a lens having a complicated shape such as that of the sub-non-arc surface, molding of a plastic material is suitable, and, thereby, it is possible to reduce the cost.
Each of the above-mentioned optical scanning devices may be of a multi-beam-scanning system in which a plurality of light fluxes are emitted from the light source, are gathered to form a plurality of beam spots on the surface to be scanned by the scanning and image-forming optical system, a plurality of scan lines on the surface to be scanned are scanned simultaneously by the plurality of beam spots.
Each of the above-mentioned optical scanning devices may be of a single-beam-scanning system in which a single light flux is emitted from the light source, is gathered to form a beam spot on the surface to be scanned by the scanning and image-forming optical system, a single scan line on the surface to be scanned is scanned by the beam spot at each scanning.
The above-mentioned optical scanning device of the multi-beam-scanning system may be configured xe2x80x98to couple the plurality of light fluxes from the light source by a coupling lens, cause the thus-obtained light fluxes to form line images each extending in the main scanning directions on a deflection reflective surface of the optical deflecting unit or in the vicinity thereof by a line-image forming optical system common to the respective light fluxes, then reflect and deflect the light fluxes at a uniform angular velocity by the optical deflecting unit, gather the reflected and deflected light fluxes so as to cause them to form a plurality of beam spots separate in the sub-scanning directions, and scan the plurality of scan lines on the surface to be scanned simultaneously using the plurality of beam spots.
In such an optical scanning device of the multi-beam-scanning system, xe2x80x98a monolithic semiconductor laser array in which a plurality of light-emitting sources are arranged in a linexe2x80x99 may be used as the light source which emits a plurality of light fluxes.
An image forming apparatus according to the present invention is xe2x80x98an image forming apparatus in which a latent image is formed on a latent-image carrying body by optical scanning, the thus-formed latent image is developed, and, thereby, a desired image is obtainedxe2x80x99. In the image forming apparatus, as an optical scanning device for performing the optical scanning of the latent-image carrying body, any of the above-mentioned optical scanning devices is used.
In this case, the image forming apparatus may be arranged in a manner such that a photoconductive photosensitive body is used as the latent-image carrying body, a formed latent image is visualized as a toner image, the toner image is transferred and fixed onto a sheet-like recording medium (transfer paper, a plastic sheet for an overhead projector, or the like), and, thus, a desired image is obtained.
Alternatively, for example, a silver photographic film may be used as the latent-image carrying body. In this case, a latent image formed by optical scanning by the optical scanning device can be visualized by a developing method of a normal silver photographic process. Such an image forming apparatus can be put into practice as an xe2x80x98optical plate-making machinexe2x80x99, for example.
The above-mentioned image forming apparatus in which the photoconductive photosensitive body is used as the latent-image carrying body can be put into practice as a laser printer, a laser plotter, a digital copier, a facsimile machine, or the like.
As described above, according to the present invention, it is possible to achieve novel optical scanning devices and image forming apparatuses. In an optical scanning device according to the present invention, as described above, a xe2x80x98sub-non-arc surface(s)xe2x80x99 is (are) formed in a lens(es) included in the scanning and image-forming optical system, and, thereby, not only image-surface curvature and optical magnification, but also wavefront aberration are well corrected. Thereby, it is possible to achieve high-density, satisfactory optical scanning using a stable, satisfactory beam spot. Further, in an image forming apparatus according to the present invention, because the above-mentioned optical scanning device is used, it is possible to achieve satisfactory image formation.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.